High frequency amplifier



Feb. 9, 1937.

W I I I I I 3 N wN v INVENTORS ED IN A. BEANE EDWARD F A REWS BY /r 2 vM ATTQRNEY l Feb. 9, 1937.

E. A. BEANE ET AL HIGH FREQUENCY AMPLIFIER Original Filed June 17, 19245 Sheets-Sheet 2 INVENTORS EDWIN A. BEANE E/DW [2 ANDREWs ATI'ORNEY Feb.9, 1.937.

E.'A. BEANE El AL HIGH FREQUENCY AMPLIFIER 3 Sheets-Sheet 5 OriginalFiled June 17, 1924 zjg. i

ll Illlllll.

NDREWS VI E Ms M .F R M D 0 WM I WW EE/ Y B Patented Feb. 9, 1937 UNHTEDSTATES PATENT OF ICE HIGH FREQUENCY AIVIPLIFIER Original applicationJune 17, 1924, Serial No. 720,549. Divided and this application March 1,

1932, Serial No. 596,029

2 Claims.

Our invention, which is a division of application Serial No. 720,549,filed June 17, 1924, issuing as Patent 2,013,430, September 3, 1935,relates to circuits and apparatus for use with thermionic tubes and tothe proportioning of the elements of said circuits and apparatus forsecuring efiiciency in the amplification of high frequency electriccurrents, such, for example, as are encountered in radio reception. Theobject of our invention is to prevent sustained or otherwiseobjectionable oscillation, to secure stability in radio frequencyamplifiers throughout a broad band or range of carrier frequencies and,at the same time, to secure efficient selective amplification withoutdistortion and to accomplish these objects in a simple and effectivemanner.

Sustained oscillation is due to a transfer of energy from the output tothe input circuits of thermionic tubes. This transfer of energy takesplace in part through the capacitative coupling between the grid andplate element within the tube, and in part through the electro-staticand electromagnetic coupling between the transformer windings, wires,and other apparatus included in the input and output circuits of thetubes. The tendency of a three element thermionic tube to become a selfexcited oscillator is a fundamental characteristic of this device, whenused in a radio frequency amplifier in which a plurality of 30 tubes arecoupled together by means of transformers. This is especially true whenthe thermionic tube in question is being used for the reception ofsignals at relatively high frequencies such, for example, as are used atthe present time 5 for broadcasting news and entertainment.

Sustained oscillation and instability occur when the amount of energyfed to the input circuit from the output circuit is such that the totalamount of energy reaching the input circuit is 40 equal to or greaterthan the total energy losses in the input circuit from all sources. Atlower frequencies below 150 kilocycles the natural tendency of anefficiently designed, transformer coupled, radio frequency amplifier tosustained oscil- 45 lation is not so great as at frequencies of 550kilocycles or above such as used at present for broadcasting news andentertainment, the tendency to oscillate increasing as the receiver istuned or designed to receive higher frequencies.

50 In a three element thermionic tube used as an amplifier, the gridcircuit must control the plate circuit. The coupling existing betweenthe plate circuit and the grid circuit limits the potential which can bebuilt up across the plate circuit 55 without so great a transfer ofenergy back to the grid circuit that the plate circuit will exercise adetrimental control over the grid circuit, thereby producing distortionand instability.

Even a transformer coupled, radio frequency amplifier, such as hereindescribed, the plate circuit impedance of which is so designed as toavoid oscillation at a given frequency, will oscillate when the gridcircuits'of the amplifier are tuned to receive a signal at a certainsufficiently higher frequency, unless the inductance of the platecircuit be lowered or some equivalent effect secured as the frequency tobe amplified is increased.

Our invention recognizes the inherent tendency toward sustainedoscillation in transformer coupled radio frequency amplifiers,especially at relatively high frequencies and contemplates limiting thisinherent tendency so as not only to prevent sustained oscillation at alltimes but also to secure stability over a relatively broad band offrequencies at which signalsare received and to which the grid circuitis tuned.

With the foregoing object in view, our invention consists inpro-portioning the inductance in the plate, or output circuit, ofthermionic tubes, used in radio frequency amplifiers of one or morestages, with respect to the frequency to be received and the variouslosses of the associated circuits, so that the frequency to which theplate or output circuit is resonant shall be at all times sufficientlydifferent from the frequency of the carrier wave being received, and towhich the grid circuit of one or more tubes are tuned, that resonance inthe plate circuit never occurs or is never too closely approached atfrequencies intended to be received on the grid circuit. Our inventionalso consists in preventing the transfer of electro-magnetic energybetween the coils in the input and output circuits of the amplifiertubes and, at the same time, securing the maximum sensitiveness andamplification in the amplifier, consistent with stable operation over abroad band of wave lengths.

One simple embodiment by which our invention can be practiced consistsin providing the primaries of radio frequency transformers, whichprimaries are connected in the plate circuits of radio frequencyamplifying tubes, with such widely spaced turns and so few of them thatthe impedance across the plate circuit will be relatively low at thehighest frequencies to be impressed upon the grid circuit of theamplifier tube, so intimately coupling this primary winding with thesecondary winding of the transformer that the tuning of the circuitincluding the secondary winding will exercise a very great where thefrequency to which the platecircuits influence upon the actual impedanceof this primary circuit, and at the same time mounting the transformersin non-axial positions in substantially parallel planes with their turnsall run- .ning in the samedirection but with the plate of the first tubeconnected to the outer end of the primary winding and the grid of thesucceeding tube connected to the outer end of the a are resonant ischanged .only-;automatically through the adjustment of the nextsucceeding grid circuit;

Fig. 2' represents diagrammatically two stages of radio frequencyamplification, whererthe fre- V 'quency' to whichthe platecircuits areresonant.

is changedby .a manual adjustmentof the-inductance'sthrough a change inthe mutual in- Iductance between primary turns brought about by changesin the spacing of the primary turns on; the radio frequencytransformers, which change in spacing is brought about mechanicallyresponsive to tuning oneor more grid circuits; Fig. 3 representsdiagrammatically a circuit arrangement similar to Fig. .1, in which theinductances of' one or 7 more plate circuits are changed through theoperation of a variometermechanically connected'to move with one or moreof thetuningdevices for the grid circuits; Fig. 4 representsdiagrammatically a circuit similar to that of Fig. '1," but in whichvariotransformers are used for adjusting the impedanceof the platecircuits'responsive to changes in adjustment of the impedance of thegrid cir: cuits; and

Fig; 5 represents a preferred form of vario transformer,

Fig. 6 represents a form of inductance found to'be suitable for use inthe system shown in Fig. 1.

Referring specifically to Fig. lythe thermionic tubes V, VI and V2 arethe amplifiers and detector in a two stage radio frequency amplifyingcircuit in which SI, S2 and S3 are the secondaries and'CI, C2 and C3,the variable condensers in the grid circuits of said amplifiers. .P andPI are the primaries located in the plate circuits of V and VIrespectively, the proper connection and winding of the transformersbeing indicated. l

In Fig. 2, the same circuit, arrangement is shown as in Fig. 1, and thesame numbers designate similar parts. The primaries P and PI areconstructed so that their inductance can be va- :ried as the capacity ofcondensers CI and C2 are varied. When the shaft 1 is rotated, this isaccomplished by the discsd and 5 moving toward {the secondaries S2 andS3, by'means of the threads.8 and or'other suitable mechanism. As thediscs 4 and 5'approach the secondaries S2 and S3, the turns in theprimaries P and PI come closer together, .thusincreasing theirmutualinductance. The condenser CI and the disc ,4 can beseparated fromthe condenser C2 and the disc 5 and operated by separate controls, if

the accompanying drawings, -in

' I 9 rotates, etc.

desired. When the connections to the transformersare made as shown inFig. 2, it is desir able that the primary and secondary turns run in thesame direction. This causes the magnetic field around SI to oppose thetransfer of electromagnetic energy to it from'P and S2 to oppose asimilar transfer from PI.

Fig. 3 is similar to Fig.- 2, except that a variometer I0 varies theinductanceof both primaries P and FL When the inductance in the primarymust be kept very low, this arrangement is not as suit.-

able as that shown'in Fig. 2 asit is desirable to keep all of theinductance in the primaries where it will be efiective in securing theproper coupling to the secondaries. A variable resistance I2 may beincluded inthe antennacir'cuit I3,

which isused only when antennas of very low resistance are employed andwhen the frequency being receivedfis very high. This re'sistance'is onlyvaried when the antenna is changed or when it is desired 'to' receivevery high frequencies.

cept at extremely high frequencies and itis not necessary even with aloop antenna of very 'low resistance at frequencies not higher. than1350 kilocycles' -With the ordinary antenna, it is not required ex- Fig.4 representsfa similar circuit to Flg'ss2] and 3, except that disc typevariometers' are used for tuning the secondaries S2 and S3 and: theprimaries P and PI. In this case as in those shown in the other figures,the direction of the primaries and secondaries relative to theconnection of their various ends should be made'so that their magneticfields" will oppose any energy transferred from PI to S2, or P to SI.The element I4'of SI is stationary while I5 rotates; the element I6 of Pis stationary while I'I rotates.

These coils of wire may be wound upon spider web forms; one coil aboveand one coil below the center as shown in Fig. 5.

, It will also be seen that the coils I6 and I8may be wound on the sameform and likewise I1 and I9 as shown in Fig. 5. This forms a flat orpancake type of variable transformer with a primary consisting of a verysmall number of turns,

half on the rotor element, and half' o-n the sta-y tor element, and asecondaryconsisting of a relatively large number of turns, half on therotor and half on the stator.

stages of amplification have been shown, it will be understood that morestages may be added if desired, or that only one stage may be used;

The operation of allof the foregoing embodiments of our invention andother embodiments coming within the scope thereof may be explained bythe following theoretical consideration, which Varying'the position of Ithe stator relative to the rotor changes the inis here included becauseof the absence in pub lished form of any concise statement of thesethe-v ories, the understanding of which is important to a thoroughunderstanding of our invention.

The sensitiveness and efficiency of a radio frequency amplifier depends,among other things, upon the impedance of the plate, or outputcircuit ofthe thermionic tubes, to the frequency of the current being receivedupon the input circuits of the tubes. If the output circuit is tuned toresonance at the frequency of current in the input circuit, theimpedance across the output circuit increases at that frequency and thecurrent in the primary of the transformer also increases. As theimpedance of the output circuit increases, the voltage across thatcircuit increases, and the energy which is transferred into the inputcircuit of the same tube is also increased.

When the impedance across the output circuit reaches a valuesufficiently great, that the energy transferred from the output circuitto the input circuit, is equal to or greater than all losses in theinput circuit, sustained oscillations and instability occur. Inasmuch asthe transfer of energy from the output circuit to the input circuitincreases as the impedance across the output circuit to the frequency ofthe carrier wave being received increases, it is apparent that byreducing the inductance in the output circuit, this circuit can bebrought sufficiently out of resonance to the frequency of the current inthe input circuit so that the voltage across the plate circuit, andtherefore th energy transferred from the plate circuit to the gridcircuit, will be limited, at least to such an extent that substantiallyless energy will be transferred to the grid circuit from the platecircuit than is dissipated in the grid circuit. The plate circuit mustbe kept sufficiently away from resonance so that it is removed by asubstantial distance from this critical condition if stability in thecircuit and freedom from distortion and noises due to more or lesssustained oscillation are to be secured.

It is, of course, desirable to maintain the current in the transformerprimary in the plate circuit as high as possible consistent with theelimination of undesired oscillation so as to transfer the maximumenergy to the secondary in the grid circuit of the succeeding tube. Itis therefore desirable to operate the plate circuit as close toresonance as possible consistent with stability over the band offrequencies to be received, in order to secure the maximum radiofrequency relay amplification. It will be seen that the fundamentaltendency of radio frequency amplifiers to oscillate, particularly athigh frequencies, presents itself as the factor, limiting radiofrequency relay amplification at these frequencies. In order to preventthis oscillation, losses must be voluntarily imposed in some manner uponsome one or more of the associated circuits so as to prevent the tubesfrom becoming self excited oscillators. It is believed that the reasonfor the greater efficiency of radio frequency amplification at the lowerfrequencies is that it is possible to operate a low frequency amplifierwith its output circuits much closer to resonance without producingunfrequency amplifiers by imposing artificial losses, or some equivalenteffect upon the grid circuit. This has been done in some cases bymeans-of a variable resistance in the grid circuit and by varying, thegrid bias.

Another general method which is susceptible to a number of variations isto transfer energy to the grid circuit of such phase as to oppose thenormal energy in the grid circuit. This has an effect which is similarto the method previously described inasmuch as it reduces the eifectiveenergy in the grid circuit thereby reducing the energy in the platecircuit. Our invention differs from all previously employed methods forpreventing sustained oscillation and securing stability in that itprevents the transfer of energy through the grid plate capacity solelyby maintaining the impedance across the plate circuit sufficiently lowso that the voltage built up across this circuit is not great enough tocause sustained oscillation or instability even though the losses in thegrid circuit are so low that the tube would immediately become a selfexcited oscillator if resonance, which is coincident with maximumimpedance across the plate circuit was anywhere nearly approached. Theoutput circuits of previous radio frequency amplifiers have beenconstructed with inductances so high relative to the frequency of thesignals which they were designed to receive and with so muchelectro-magnetic coupling to the input circuit, that they would normallyoscillate violently, unless prevented from doing so by the introductionof some device for producing losses or an equivalent effect in the gridcircuit. In our invention the inductance in the plate circuit is notonly reduced to the critical point at which oscillation is justprevented Without the introductionof artificial losses in the gridcircuit and in which condition a disturbance will throw the circuit intomore or less sustained oscillation but the inductance of the platecircuit is reduced sufficiently to bring the amplifier substantiallypast this critical point so that stable operation is secured over arelatively wide band of wave lengths without the use of a separatelyoperated control for maintaining this condition.

The reduction of plate circuit inductance which we employ to preventmaximum impedance across the plate circuit, also reduces the magneticfield around the primary of the transformer in the plate circuit therebyreducing the tendency for electro-magnetic energy to be transferred tothe secondary of the preceding transformer in the grid circuit of thesame tube. The transfer of electro-rnagnetic energy from the platecircuit to the grid circuit is further prevented by the arrangement ofthese circuits so that their respective magnetic fields oppose thetransfer of energy from one to the other.

We believe our method for accomplishing the above object to be superiorto that in which energy of reverse phase is transferred to the gridcircuit as proposed by Hartley in Patent No. 1,183,875, Rice in PatentNo. 1,334,118 or Hazeltine in Patents No. 1,450,080 and No. 1,489,228 inthat it is more simple, more direct, easier to construct and control andalso for other reasons among which is that by our method the originaltransfer of energy is prevented instead of a transfer occurring andbeing afterwards neutralized by a more or less equal and oppositeelectromotive force. resistance or grid biasing method above referred tofor the following reasons:

Our method is also superior to the grid 7 gIf lo'ssesintrodu'ced toprevent sustained oscillation take the form of resistance, theynecessarialy increase the damping. ofthe circuits, broadeningtheirtuning and making it difiicult to separate signals of slightly differentfrequencies. For this,'and also for other reasons, it is far lessobjectionableto introduce any losses'necessary' to prevent sustainedoscillation by varying the reactance rather than by increasingresistance. We have also found that it is preferable to introducethese'reactance losses in the plate circuit.

There is a larger amount of energy in the plate circuit which permits ofthe use of larger values for its control. Therefore, the balancing ofvalues for accomplishing the desired result without the introduction ofexcessive losses is far less critishould only be adopted after themethod herein cal.

Although it is desirable to reduce the resistance of all circuits,employed in the amplifiers herein described, a certain amount ofresistance is nevertheless unavoidable. This is not alto- "getherobjectionable as it would be very difficult, 'if not impossible, tosecure stable operation in such an amplifier were it not for thepresence The lowerthe of some-appreciable resistance; resistancebecomes, particularly of the grid'circult, the greater the variation ofthe plate circuit .reactance required tosecure stability. Also thehigherthe frequency of signal being received, the greaterthe energytransfer from the plate cir-, cult to the grid circuit through a givencoupling.

It is conceivable that at. very high frequencies and with all resistancelosses reduced to a very low point, a condition might be arrived at,under which the above described method would not be adequate forsecuring stability. Should such a condition beencountered, 'we haveshown means by which it can be overcome by placing a variable resistancein the antenna circuit. It must beborne. in mind, however, that thisexpedient described for preventing oscillation has been ap plied as faras possible, so as tomake necessary the introduction of only thesmallest possible amount of resistance to secure the desired degree ofstability.

We have found experimentally, however, that at frequencies of from 1350to 550 kilocycles, it is possible and desirable to reduce all losses,except plate circuit reactance losses, to the lowest practicable values,and still prevent undesired oscillation by the method of reducing, platecircuit inductance as herein described, even when loop antennas of verylow resistance are employed as collectors.

A further feature of this invention is the maintenance of the radiofrequency amplifier herein dealt with in its most sensitive conditionconsistent with stable operation while it is tuned to receive signals ofdifferent frequencies covering a relatively wideband such as the variousfrequencies now being used for broadcasting news and entertainment. Itis a well known fact that previous radio, frequency amplifiers of thistype tend to oscillate more and re- ,quire the introduction of greaterlosses to preadvantageous :vent this oscillation when they are tuned toreceive signals at higher frequencies than when they are tuned to lowerfrequencies.

It has been supposed that this was due to a more inductance capacityratio these frequencies in' the grid'circuit resulting in a higherpotential across the grid circuit. We have discovered, however, that thegreater tendency to oscillate at the higher frequencies is not idue'inany large measure to the above mentioned cause but principally to thefactthat; the transfer of energyfromthe plate circuitto the grid circuitincreases as the frequency increases,

There are two reasons-for this: first, a greater amount of energy can;be transferred through a given coupling; other conditions remaining thesame, at a higher frequency than at a lower frequency, and second, whenthe platev circuit is slightly above resonance at a lower frequency, andthe grid circuit is then tuned toreceive a.

signal'at a'higher frequency without any change of the values connectedin the plate circuit, the

plate circuit will be'c'lcser to resonance at this higher frequencywhich will result in a higher voltage across it and a greater transferof energy to the grid circuit. If a radio frequency amplifier of thetypedescribed' is to be, maintained in the desired state of maximumvsensitivity and responsiveness to weak incoming signals,'consistent withstable operation over a relatively broad band of frequencies, theimpedance of the plate circuit to the higher frequencies shouldbegreduced as the grid circuit is tuned to receive, higher'frequenciesthus preventing an increase in the trans fer of energy'from the platecircuit to the grid circuit at the higher frequency. Y

This can be'done in two ways; first,,by reducing the physical inductancein the plate circ'uit'as the frequency of the signal beingr receivedincreases, and second, by coupling the plate circuit of one tube to thegrid circuit of, the succeeding tube as closely as possible. This lattermethod, while very useful, does not'with the degree of coupling so farobtained preventv the tendency towardoscillationincreasing slightly asthe free-- However, as long as the :an-

quency increases. 7 tenna resistance is fairly high, very satisfactoryresults can beobtained by this method. We have also found another effectwhich can also be made to act against the greater tendency towardoscillation with increases in frequency. This 'canbe accomplished byarranging theconnections and turns in the 'coils containedin the outputand input circuit of an amplifier tube in, such a way,

that a very small amount of energy is transferred electromagnetically'from the output cir-- tion of this invention, but constitutes a means ofopposing the increase in the tendency toward oscillation at higherfrequencies which may be useful under certain special conditions.

There is still another method by which the increase in the tendency tosustained oscillation and instability as the frequency increases can beminimized or eliminated. This consists of placing a greater amount ofinductance or capacity inthe plate circuit than the amount required, to

produce resonance at the frequency being received. ,Afpoint at which theplatecircuit is not resonant to the frequencybeing received on the gridcircuit can be thus reached by adding to the inductance in the platecircuit as well as subtracting from it. If this inductance is increasedsufiiciently above resonance so that oscillation is prevented at thelowest frequency to be received, tuning the grid circuit to andreceiving a signal of higher frequency will now bring the plate circuitstill further away from resonance than before thereby decreasing thepotential across the plate circuit and tending to decrease the transferof energy to the grid circuit. This effect would then work against thegreater transfer of energy, through a given coupling at higherfrequencies. In this way the two effects may be made to more or lessbalance one another thereby minimizing or eliminating the increase inthe tendency toward oscillation as the frequency increases. There are,however, two objections to this method. First, the increase in theinductance across this circuit and it is conceivable with a large coilthat an appreciable voltage might be built up across the plate circuitdue to the impedance of the inductance coil. This could be minimized byusing as small a coil as practicable with a large condenser across it.Second, this method necessitates having a large inductance in the platecircuit, as large or larger than in the grid circuit of the succeedingtube.

Unless the inductance in the plate circuit was separated into twoinductances, only one of which was used for the transformer primary astep down transformer would result instead of a step up transformer withmore secondary turns than primary turns. A step up transformer isfurnished naturally by the type of transformer in which the inductanceof the plate circuit is reduced by reducing the primary turns. This stepup transformer effect is highly desirable as it considerably increasesthe voltage amplification over lower ratio step up transformers or othercoupling means which do not provide in themselves a high voltageamplification, it also secures a. high degree of selectivity.

It is an essential feature of this invention to reduce the couplingbetween the plate circuit and the grid circuit of the amplifier tubeswhich tends to produce undesired oscillation, either by the methodherein described or by the reduction of the physical coupling of thesecircuits through the tubes, transformers and associated circuits to thelowest point possible. The more completely this coupling is reduced, theless reduction will be necessary in the inductance of the plate circuitsto prevent undesired oscillation, and inasmuch as the reduction of platecircuit inductance also increases the plate circuit reactance losses atthe frequency being received and to which the grid circuit is tuned, andas it is desirable to maintain these losses as low as possibleconsistent with staple operation, it will be seen that great care shouldbe taken to avoid the reduction of plate circuit inductance beyond theamount necessary to secure stability. It is also desirable to avoid theunnecessary reduction of plate circuit inductance for the reason that itreduces the electro-magnetic coupling between the plate circuit of onetube and the grid circuit of the succeeding tube. It is desirable tomaintain this coupling quite close, at least in the form of thisinvention where the physical inductance in the plate circuit is notvaried.

By connecting the proper end of the primary coil in the plate circuit ofa tube in an amplifier of the type described, to the plate, withrelation to the end of the secondary coil in the grid circuit of thesame tube which is connected to the grid and by maintaining this properconnection in all the coils and by fixing the coils in the properposition, a condition can be secured by which the field of anytransformer, or

vario-transformer used in the amplifier can be made to oppose the fieldof the transformer on either side of it to just the proper degree sothat the fields of these transformers will not interlink and there willbeno transfer of electromagnetic energy from the plate circuit to thegrid circuit of the amplifier tubes. This condition of zero magneticcoupling is the desired condition and when this is attained it is onlynecessary to compensate for electrostatic coupling between the plate andgrid circuit by reducing plate circuit impedance. We have found withflat spider web transformers four inches in diameter with sixty-threeturns in the secondary and six to eight turns in the primary, that goodresults are obtained when the transformers are placed parallel to oneanother on a common axis with a separation of approximately six and onequarter inches. When all the turns on all the transformers run in thesame direction, all the rids should be connected to the outside of thesecondary winding and all the plates to. the outside of the primarywindings. If the coils are put at right angles to one another or if oneof them is turned through degrees without at the same time turning theother two, sustained oscillation will immediately result and can only bechecked by a very great reduction in the number of primary turns. It ispossible to also place these coils in non-axial positions in parallelplanes and still maintain the desired condition, but if the coils areplaced end to end in the same plane, even with a considerable spacing,oscillation will commence.

The primary turns may be wound if desired, in the opposite directionfrom the secondary turns. Under these circumstances theinner end of theprimary windings should be connected to the plate, all other conditionsremaining the same. By placing the transformers at the proper distance,one from the other and rotating them simultaneously to more or lessaxial positions,

the desired condition of maximum stability can be obtained. This canlikewise be accomplished when the coils occupy axial positions byvarying the distance between them.

Relative to the embodiment of this invention in which the physicalinductance of the plate circuit is not changed with the frequency, it isimportant that the inductance of the output circuit be made such as toavoid objectionable oscillation at the highest frequency to be received.It is also desirable to impose the least possible reactance losses inthe plate circuit at the frequency of the signals being receivedconsistent with stability.

It is therefore desirable to produce a transformer which shall have theclosest possible coupling between the primary and secondary and at thesame time maintain a low primary inductance. We secure thesecharacteristics by constructing our air core transformers with a smallnumber of primary turns which are separate one from another preferablyby being interwoven with the secondary turns, so as to space the primaryturns and thereby reduce their mutual inductance and distributedcapacity. Any number of secondary turns desired may be wound between oneprimary turn and the next. I

This arrangement secures a very intimate relation of the primary to thesecondary turns and makes it possible to secure a greater length ofprimary turns in direct contact with the secondary turns, at the sametime maintaining the minimum primary inductance consistent therewith.

1 -To take full adv'antage'of our invention all electrical losses exceptplate circuit losses pure posely imposed should be reduced as far aspossible. We therefore prefer the spider web type of coil fortransformers with a minimum of dialectic material necessary to pport thewire.

We have devised a form of spider with a greatly reducedamount ofdialectic-material and also an easyway of winding these transformersconsisting of winding the primary and secondary tur-ns together as ifthey were one wire, then cutting oif the primary at the desired pointand continuing the secondary to the outside of the spider.

We have found that with the form of this invention in which the physicalinductance in the plate circuitis not changed, that when thetransformersare so constructed that the reactance lossesof the platecircuit containing the primary of the transformer {is as low as possibleat the lowest'frequency to be received, consistent with stable operationthat when theamplifier is tuned toafhigher' frequency (achange forexample of from 550 b01350 kilocycles), the tendency to oscillate isgreater at the higher frequencies.

'Our invention, therefore, contemplates, where desired, means forvarying the physical inductance in the plate circuit so that it becomesprogressively loweras the amplifier is tuned to re-' By a slightvariation;

ceive higher frequencies. of the very small inductance in the platecircuit as the grid circuit control is adjusted, the am --plifier maybemaintained in that desirable state just sufiiciently below the criticalcondition in the plate circuit previously referred to while the gridcircuit tuning is changed over a relatively broad band of frequenciessuch, for example, as are now in use for broadcasting music andentertainment.

This changing of the frequency to which the;

plate circuit is resonant by changing the physical values connected intothe plate circuit can be accomplished either by a variable condenser orby a variable inductance either of which can be operated by a separatecontrol or by being connected to the controls used for tuning any or allof thegrid circuits. Where this form of the invention is employed, weprefer to use a variable inductance which constitutes the primary of anair core transformer and this inductance is va-f comprising a. V

circuit, including a" coil magnetically coupled to the first coil,connected between the input electrodes of theseco nd tube, said coilsbeing so wound that energy is notfed back from the'out put circuit ofthe first tube to its input circuit,

and unitary means for simultaneously tuning said secondary circuit,adjusting the self-inductance of each coil, and varying the magnitude ofsaid coupling.

2. A radio frequency amplifier comprising a pair'of cascadedmulti-electrode tubes, a primary circuit, including a coil, connectedbetween the output electrodes of the first tube, a'secondary circuit,including a coil magnetically coupled to the first coil, connectedbetween the inputelectrodes of the second tube,-said primary circuit 1being maintained resonant to a frequency greater than the frequency towhich the secondarycircuit is tuned, said coils being so wound thatenergy is not fed back from the output circuit of the, first tube -toitsinput circuitand unitary means for simultaneously tuning saidsecondary circuit, adjusting the self-inductance of each coil, andvarying the magnitude of said coupling.

EDWIN'A. BEANE. 1 EDWARD F. ANDREWS.

